Tuesday, September 29, 2009

Dark Matter Hunters Construct a New Weapon by Brandon Keim

That dark matter has never been found is no deterrent to the physicists who are looking for it.“Even if we don’t know what dark matter is, we know how it must act,” said Eduardo Abancens, a physicist at Spain’s University of Zaragoza and designer of a prototype dark matter detector.According to physicists, only around five percent of what makes up the universe can presently be detected. The existence of dark matter is inferred from the behavior of faraway galaxies, which move in ways that can only be explained by a gravitational pull caused by more mass than can be seen. They estimate dark matter represents around 20 percent of the universe, with the other 75 percent made up of dark energy, a repulsive force that is causing the universe to expand at an ever-quickening pace.At the heart of Abancens’ team’s detector, which is called a scintillating bolometer and resembles a prop from The Golden Compass, is a crystal so pure it can conduct the energy ostensibly generated when a particle of dark matter strikes the nucleus of one of its atoms.To prevent interference by cosmic rays, the bolometer is sheathed in lead and kept underground, under half a mile of rock. It’s also frozen to near-absolute zero, the temperature at which all motion stops. At the edge of absolute zero, it’s possible to measure expected changes of a few millionths of a degree Fahrenheit.Researchers like Abancens call this “a high heat signal.”As described in a paper published in the August Optical Materials and released online Friday, the bolometer is currently able to distinguish between the vibrations produced by trembling nuclei and spinning electrons.Abancens said it could be operational in five years.But in order for the bolometer to work reliably, it needs to become even more sensitive, and maintain that sensitivity as it’s scaled up from the 46-gram prototype to a half-ton working model, said Rick Gaitskell, a Brown University physicist who was not involved in the research.At near-absolute zero, conducting research is “quite challenging,” said Gaitskell, who spent a decade trying to make detection systems work at that temperature.“Now we’re using using liquid xenon. It’s relatively warm, only minus 150 degrees Fahrenheit,” he said. “You can nearly get to that in an industrial-strength refrigerator.”

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